This invention relates to an amplifier suitable for amplification of infrared signals detected by photovoltaic detectors and, more particularly, to a construction of amplifier employing a charge-coupled device with high-speed alternate sampling of a detector signal and a detector noise level. Noise is correlated in the resulting sample sequences. An output signal is obtained as the difference of integrals of both of the sequences.
There is considerable interest in the construction of infrared scanning systems to provide images of infrared subject matter. Typically, such systems employ an array of photovoltaic detectors which is scanned across subject matter to develop a set of swaths from which an image of the subject is obtained. In order to reduce the size of the imaging system, it is convenient to construct the array of detectors from semiconductor material disposed on a common substrate. In addition, to minimize the use of numerous wires for connection with individual ones of the detectors, it is preferable to build a multiplexer on top of the detector array whereby individual ones of the detector signals can be sampled and sequentially switched via the multiplexer for communication via a common link to a utilization device. The utilization device would include a demultiplexer, filters, and other well-known circuits for combining the detector signals to display an image of the subject.
A set of preamplifiers is connected between respective ones of the detectors and the multiplexer. Preferably, the amplifiers would be constructed on top of the array of detectors alongside the multiplexer, thereby to reduce the overall size of the detector assembly.
A problem arises in that the amplifiers must be capable of amplifying the individual detector signals without the introduction of excessive noise, thereby to enable the presentation of a clear image of the subject. In the detection of infrared radiation having a wavelength of 12 microns, it is advantageous to employ a detector composed of mercury, cadmium, and tellurium.
One solution to the problem of noise reduction is the use of low temperature operation in which a plane of the detectors, often referred to as the focal plane, is cooled to a temperature of approximately 851 Kelvin. Clearly, the introduction of such cooling greatly increases the complexity of the scanning system. In addition, the cooling introduces electrical effects such as an increase in the impedance of the detectors. In the case of the foregoing mercury-cadmium-tellurium photovoltaic detectors, the noise is characterized as a drift in the magnitude of the quiescent current of the detectors. Such drift derogates from the operation of the scanning system, but can be significantly reduced by a lowering of the temperature of the focal plane. However, as noted above, such low temperature operation is disadvantageous because of the excess complexity associated with cooling equipment.